The oral cavity is the portal of entry for many infectious pathogens. Despite advances in the field of mucosal immunity, mechanisms of immunity in the oral mucosa remain surprisingly poorly understood. Candida albicans is a commensal fungus that commonly colonizes mucosal surfaces including the mouth. In healthy individuals, Candida is non-pathogenic. However, in immunodeficient patients, such as those with HIV/AIDS, Sjgren's syndrome, congenital immunodeficiency or those receiving chemotherapy, this microbe causes severe opportunistic infections of the oral cavity, known as thrush or oropharyngeal candidiasis (OPC). We recently discovered that immunity to OPC is highly dependent on IL-23, IL-17, and ROR?t, overturning the long-held paradigm that immunity to Candida is mediated by Th1 cells and IFN?. Our findings have been validated in in patients with rare immunodeficiency diseases that impact the IL-23/IL-17 pathway and who suffer from OPC and other forms of chronic mucocutaneous candidiasis (CMC). For example, Hyper-IgE/Job's Syndrome (HIES) is caused by mutations in STAT3, a transcription factor downstream of IL-23 and other cytokines. HIES patients have low levels of Th17 cells but not Th1 cells, and usually suffer recurrent OPC/CMC. Similarly, rare IL-17R-deficient families present with OPC and CMC. In this proposal, we will address several unanswered questions regarding oral immunity to Candida, emphasizing the IL-23/STAT3/IL- 17 pathway. It is clear there is a powerful innate response to Candida, but the nature of this cell type is unknown. Using new reporter mice, in Aim 1 we will determine the nature of the key IL-17-producing innate cell type that provides the early response to this organism. In Aim 2, we will evaluate mechanisms of immunity to OPC controlled by STAT3, particularly in oral epithelial cells. In Aim 3, we will evaluate the mechanisms by which IL-23 and STAT3 signaling regulate the adaptive anti-Candida Th17 response. Together, these studies will define the roles of STAT3, IL-23 signaling and IL-17 in the maintenance of oral mucosal immunity.